A power amplifier (PA) for mobile application is one of the largest power consuming components used in handsets and other mobile devices. Considering the nature of handheld devices and mobile devices, a PA needs to have (1) high power-added efficiency (PAE), (2) low idle current, and (3) low spectrum leakage. The first two features are required for longer battery life while the last feature needs to be ensured for co-existence with other wireless standards. The standard Wideband Code Division Multiple Access (WCDMA) system employs non-constant envelope modulation and frequency division access. As a result, a WCDMA PA needs to be linear enough to support a non-constant envelope and has to have low idle current since frequency division access needs the PA to be on for the whole talk time or use time unlike the time division access system. In order to preserve battery life, a portable WCDMA PA is recommended to have three power modes: low-, middle- and high-power mode. With a detailed level of power mode, a WCDMA PA can reduce power consumption through the excess power being cut down based on the information provided from baseband.
PAs used in WCDMA mobile handsets have tough requirements so III-V semiconductors such as gallium arsenide (GaAs) have been major candidate for power amplifiers due to their superior power handling performance in contrast to those implemented using a silicon-based process like CMOS. However, CMOS provides the feasibility of integration with baseband and other circuitry that are already designed on a CMOS process, which lowers cost and gives a competitive edge to the manufacturer. Notwithstanding, CMOS suffers from an intrinsic low transconductance, low breakdown voltage and high knee voltage, all of which make the design of CMOS power amplifiers challenging. Accordingly, there is an opportunity in the industry for systems and methods for CMOS power amplifiers with power mode control.